Linear actuator



. m9- @AKN 2 Sheets-Sheet l L. A. AMTSBERG ETAL LINEAR AcTUAToR May 30, 1950 Filed July 17, 1945 lNvEN-RS LEJTER A. MTJBERG JOHN E. OLSON BY May 30, 1950 1 A. AMTs'BERG ETAL 2,509,409

` LINEAR ACTUATOR Filed July 17, 1945 2 Sheets-Sheet 2 INVENTORS EJTER ,4 MTJER :10H/Evy E. OLSON ATTORNEY atented May 30, 19500 LINEAR ACTUATOR.

Lester A. Amtsberg-'and John E. Olson, Cleveland.

Ohio, assignors to Chicago Pneumatic Tool` Company, New York; NL. Y., a 'corporation' of New Jersey Application July V1.7, 1945, Serial"No. 605,549

(Cl. 192-150l B-Clams. l

rThis invention relates to motor-operated linear actuators, for operating the various' control and 'retractable' landing gear' parts` of an 'airplane' and 'more particularly to a load-limiting device for opening the motor circuit at the ends 'of the stroke effected by the actuator.

Heretofore, aircraft actuators have employed a torque limiting device in the gear train which is effective to open the limit switch when' a pre'- determined torque has been reached. Such a device is complicated, subject to 'maladjustment due to wear of its parts and inherently'inaccurate as a measure of the applied force because the gear train torque is affected by variations in the friction on the jack-screw.

It is accordingly among the objects of the present invention to provide a load-limiting 'device for linear actuators which is simple in construction and not subject .to maladjustment` due to wear, and which Vdirectly measures the axial force within close limits and eliminates errors resulting from j ack-screw friction.

According to the invention, a component of the axial force is transmitted to a spring through a pivoted mounting lever. The spring is preloaded to resist the component ofV the normal axial force running through the jack-"screw, and, by means of loose plates at the opposite ends' of the spring, the spring resists va force in either of two directions. on the airplane abutted by the work: member causes the axial force to build up. A part of the reactive force is transmitted through the lever Yto the spring. When the transmitted part of the force exceeds the pre-'load `of the spring, the spring is compressed Vand the motion of an actuating rod within the spring 'operates the limit switch. The overtra'vel of the motor is absorbed by the. spring. The spring is deflected and axial force is built up slightly above the pre-load value but well within predetermined limits. One advantage of the spring is that itprevent excessive stress from beingY setup in the linkage Operated by the actuator.

A reversible electric motor turns the jackscrew in. one direction or theother to Aextend or withdraw the jack-screw sleeve. Manual switch -means is used to select the motor direction of rotation. The limitV switch cuts thefmotor circuit at the ends of the stroke of .the jack-screw sleeveormember as itis lextended or withdrawn.

For other objects anda better understanding of the invention, reference may be had to the following.' detailed` description taken, in connection with the accompanyingdrawings, in which:

At the end of each strokaa stop 3.

'or bracket I! anda lever arm I 2'.

Fig. 1 is a longitudinalfsection of an actuator embodying the'features vofvv theA present invention;

Fig. 2 is a rearlrview, inelevationr ofthe actu'- ator;

Fig. 3' is a sectional vviewofthe gear reduction portion of theactuator, taken along-'the'l'ine 3-3 oFig. 1, Yand with the cover lplate thereof -removed;

Fig.' Il" is a sectional viewf'ta-ken-along the-line 4 4 of Fig. 3;

Fig. *51 is a wiring diag-ram of the electric control switchesandmotor;

Figs. 6i 'and' 7 are respectively fragmentary views, partly in section 'of 'the jack-screw sleeve andthe j ack-screw;

Fig. 8 is a top plan `view of the limit switch;

Fig. 9 isf-an' elevational view' of-'a contact' rol-ler; and

Fig. l0 isa 'pla-n View of the4 sliding insulating block which'r carriesthecontact' roller.

Referring now particularly toV Fig. 1, II) 'is the linear actuator connected between anchor point Ihe' leverann I'2 is splined to Ia shaft I3 which may be connected to some work element or part of the airplane'to be operated'by the actuator. The operate'd'part may be some controlpsurf'ace of an airplane or it may be a tail skid or'other landing gear.

The Vactuator I'' comprises generally'a jackscreW I4, a gear train t5, an electric motorA I6, a pre-loaded spring I 'I, a limit' switch I8, anda pivoted lever I9 for connecting the'actuator'y to 'the anchor 'point or bracket I'I'. 'The 'jackescrevv I 4 operates an internally threaded j ackescrew sleeve or member'ZI to adjust' thesleeve either outwardly or inwardly to lengthen or shorten the over-all` longitudinal dimension of the actuator. As shown in Fig. '6., the threaded' portion of the sleeve extends onlya short distance as shown at 2|. As the jack-screw is turned in one direction the lever armV l2 and shaft I 3 are turned through an angle in a corresponding direction,

,and if the ,jack-screw is rotated inthe opposite placement of the shoulder 26 it is welded to the casting at 26. This sleeve guide 25 thus remains stationary in the housing 21 as the internally threaded jack-screw sleeve 2| is moved in and out by the jack-screw I4. Threadedly .connected to the outer end of the sleeve guide 25 is a retaining cap 28 containing a felt wiper ring 29 serving to wipe the exterior surface of the internally threaded jack-screw sleeve 2|.

The main housing 21 is fashioned at one end to contain the reduction gearing I5. Secured to this enlarged end of the main casting 21 is a gear case or cover 3| adapted to contain a ball thrust bearing 32 for supporting the inner end of the jack-screw I4. The thrust bearing 32 is retained in the gear case 3| by an end bracket plate 33 and fastening screws 34. The gear case cover 3| is fastened to the enlarged end of the main casting 21 by bolts 35.

The end of the jack-screw I4 has a shoulder portion 36 cooperating with an inner race 31 of the ball bearing element 32 to prevent its outward axial displacement within the housing 21. A large gear 38 is splined as indicated at 39 to the jack-screw I4 and is retained thereon against the inner race 31 of the ball bearing 32 by a lock nut 40 and lock washer 4I. Also, by means of this nut and washer the jack-screw is fixed against inward axial displacement within the housing.

Referring now to Fig. 4, the arrangement of the individual gears of the gear train I5 will be described. Cooperating with the large gear 38 on the jack-screw I4 is a pinion 42 of a combined pinion and large gear 49 connected by shaft projections 44 and 45 respectively to a ball bearing element 46 in the main housing 21 and a roller bearing element 41 in the gear case 3|. The gear 43 in turn meshes with a pinion 48 of a combined pinion and gear 99 having shaft projections 5I and 52 journalled respectively in a roller bearing element 53 in the main housing 21 and a ball bearing element 54 in the gear case 3|. The large gear 49 is driven from rotor pinion shaft 55 of the electric motor I6. The gear reduction of the gear train is such as to give the desired force and Speed to the jack-screw I4.

The electric motor, which is reversible, may bc of the 3-phase induction type including stator windings 51 and a laminated stator core 58 secured to the main housing 21 by means of bolts 59. In a commercial embodiment of the invention it has been found desirable to use a squirrel cage motor operating on a frequency of 400 cycles per second and at a synchronous speed of 24,000 R. P. M. The main housing 21 has a housing portion 6| in which the motor parts are housed. Through a side hole opening 62 lead wires 63 from the stator windings 51 may extend into a limit switch housing portion 64 having a cover 64'.

In the inner end of the motor housing portion 6| adjacent the pinion end of shaft 55 is a recess 65 which contains a ball bearing 66 for supporting the pinion end of the rotor shaft 55. The shaft is retained against inward displacement by means of a shoulder portion 61 thereon which abuts the ball bearing 66. Outwardly of the shoulder portion 61 is another shoulder portion 68 against which a rotor core 69 is disposed. This core 69 is in vertical alignment with the stator core 58 to cooperate therewith. A squirrel cage ring 1| is embedded in the rotor core 69. The outer end of the pinion shaft 55 is supported in a ball bearing 12 which is housed in a motor housing cover 13. The cover 13 is secured to the main casting 21 by bolts 14.

The pre-loaded spring or lever retaining means I1 is partly disposed in a recess or housing portion 16 located on the main casting 21 immediately above the gear train I5. Over the outer end of the spring I1 and adapted to hold the spring I1 in its pre-loaded state is a spring retaining cup or case 11 having an outwardly flanged open end 18 adapted to be secured to the main casting 21 and about the recess 16 by fastening screws 19, Fig. 2.

Within the spring I1 is an actuating or push rod 8| for operating the limit switch I8. This actuating rod is of two-part construction so that the length of same may be adjusted to fit the retaining case 11. One part 82 is in the form of a yoke having connecting clevis projection 83. The upper end of the lever I9 is connected to the clevis 83 by pin and cotter means 84. Also, in this yoke part 82 there is formed a threaded recess 86 adapted to adjustably receive a second part 81 of the actuating rod 8|. This part 81 is threaded and is adjustable in the threaded recess 86 of the part 82. The part 81 also has clevis portion 88 to which an insulating member B9 is attached by means of a pin 9|. The clevis 88 extends through an opening 92 in the bottom of the recess 16 so as to connect with the insulating member 89 within the limit switch housing portion 64.

At the opposite ends of the actuating rod 8| and adapted to respectively engage the ends of the spring I1 are spring plates 93 and 94. The spring plate 93 is normally retained against the bottom of the recess 16 by spring I1 but can be moved outwardly when the actuating rod 8| is moved to the right to compress the spring I1 against the opposite spring plate 94. The outward movement of the plate 93 is effected by the clevis projection 88 of the part 81.

The spring plate 94 is normally retained against outward displacement by an inwardly extending ilange 96 on the spring case 11. The spring plate 94 also engages at its inner periphery a shoulder 91 on the actuating rod part 82. This shoulder 91 in addition to holding the plate 94 also retains the inner periphery of a flexible covering diaphragm 98. The outer periphery of the iiexible diaphragm 98 is retained against the flange 96 of the spring case 11 by an angular retaining ring 99.

As the actuating rod 8| is moved to the left the spring plate 94 is moved by the shoulder 91 of part 82 away from the inturned flange 98 to force the spring I1 against the bottom of the recess 16. The clevis portion 88 of the part 81 will pass through the plate 93 and move the insulating block 89 to the left.

The insulating block 89 carries a metallic contact roller |0| adapted to normally bridge contact plates |92 and |03. A tensioned Spring |04 serves to maintain the contact roller |9| in engagement with one or both of the contact plates |02 and |03 to connect the contact plate or plates through the roller |0| and spring |04 to ground.

Referring now to Fig. 5, there is shown a wiring diagram including the limit switch I8, a manually operated switch arm |06 connected with a direct current source wire |01 and adapted to be connected either to a contact terminal |08 or a terminal |09. When the arm |06 is on the terminal |08, a relay coil is energized. One wire of vthis relay coil is connected to the contact plate |02 of the limit switch. With the contact roller |0| in the neutral position, bridging the two contact plates |02 and |03, the circuit through the relay will be completed from plate |02,

atomes through roller I I and grounded'contact spring I 01|.V The relay III operates three relay armatures II2 to establish -a circuit from a threephase power supply I I3 with the stator windings 5'I. This establishes a revolving magnetic eld within the stator core 58 so that the motor is rotated in one direction. The motor I6 will con-- tinue to operate in this direction until the contact roller IGI leaves the contact plate |02. Whereupon, the direct current relay circuit is disconnected, the armatures I2 are released, and the electric motor I6 is stopped.

If the switch arm |06 is moved to the contact point |09, a relay II4 is energized. A circuit is established through contact plate |03, roller |9| and grounded contact spring |04. A set of relay armatures IIS is operated and the connections with the stator winding 51 are then such that the motor I8 is rotated in the reverse direction. This rotation of the motor will continue until either the switch arm |06 is manually Withdrawn from the terminal |09 or until the contact roller lill is automatically withdrawn from plate |03, whereupon the relay II4 is deenergized.

The jack-screw Itis thus operated inonevdirection or the other so as to extend or withdraw the threaded sleeve 2i. When the sleeve 2| is moving in extension vfrom the jack-screw I in normal operation to actuate the shaft I3 (or: when the jack-screw is extending from the sleeve and the normal operation is to reverse the movement of the shaft I3), the pre-loaded spring I'I will offer sumcient resistance to the reactive force transmitted from the jack-screw to maintain in stationary position the main housing 27 and the spring I' and lever i9 carried by the housing. If the action is to extend the sleeve 2 I, this movement continues until the work arm I2 engages alstop IIS. The jack-screw will continue to rotate but in doing so will be pushed rearwardly from the now stationary internally threaded sleeve 2| thereby to cause lever I9 to be pivoted rearwardly about a pivot pin III by which the lever isV connected to the anchor bracket I. The rearward thrust acts upon lever I9 at a point above the pivot pin I I'I and through a connection made by a pin I I3 and Cotter IIS with a bracket |2| on the bracket plate 33.

From the position shown in Fig. 1, the arm I2 and shaft I3 move clockwise and the connecting head 23 moves upwardly as Well -as to the left. Such upward movement causes the main housing 21, lever I9 and push rod 8| to swing as a unit clockwise about pivot pin III on the anchoring bracket |I. As the jack-screw pushes the sleeve 2| forward, it transmits an equal and opposite reaction through lock nut 40, lock washer 4I, large gear 33, inner race, balls and outer race of thrust bearing 32, cover plate 33, screws 34, gear case 3i, bracket |2I, pivot pin ||8, lever I9, pivot pin Ill to anchor bracket II. The back- Ward force transmitted to the anchor bracket II is equal and opposite to the force of the connecting head 23 on the lever arm I2. Due to the fact that the pivotal connection H8 of the lever I9 is not in alignment with the jack-screw and pivot pin Ill, the reactive force which is transmitted through the lever I9 also has a tendency to rock the lever clockwise about its pivot I|8 and exert a pulling force on the actuating rod 8|. As has been previously pointed out, the force developed by the jack-screw during normal operation, that is .before the shaft I2 is caused to abut the stop IIB during clockwise movement or stop |2| Vduring'V counter clockwise movement, is insuiicient to move the actuating rod.8 I relative to the housing 2l dueto the pre-compression or loading of spring I'I which resists suchrelative movement.

InV other words, after the sleeve has been extended by the jack-screw to vengage stop IIS the jack-screw, as it continues to rotate under the actuation ofthe motor, will be forced to move to the right carrying with it the entire housing including the pre-loaded spring II and the rod 8|. It will also carry with it to the right the lever I9 which, it is to be noted, is anchored to the bracketv I I'. This will cause the lever to turn clockwise about the pivot pin I'I as a fulcrum, and it will'also cause the longer arm of the lever to-swing to A'the right at -a greater rate than the housing. Accordingly, the lever will exercise a pull on the rod 8|, and the arrangement of the membersvis such that the predetermined pull of the rod will overcome the resistance of the preloaded spring to move the rod to the right, thereby actuating the switch.

The amount of the pulling force is considerably less than the force pushing back on the anchor.

bracket I I and corresponding to the forward push of the jack-screw. The ratio between the pull of the lever on the actuating rod 8| and the push of the jack-screw may be expressed by the fraction a/b, where a is the perpendicular distance between the pivotal connection IIS and the axis of the jack-screw; while b is the distance between the pivotal connection |I8 and the axis of the actuatingV rod 8|. It will be noted that in the present illustrative embodiment of the invention the distanceb is about four times as large as the distance a. Therefore, the lever I9 pulls on the actuating rod 8| with a force which is only about one quarter as vmuch as the pushing force of the jack-screw against the driven object.

The maximum pushing force of the jack-screw can be'determined by selecting the desired preload'ing pressure on the spring. If the designer desires to have the motor stop automatically upon attainment of a jack-screw pressure of 1600 pounds, he specifies a spring which may be preloaded to the extent of having an initial compression of 400 pounds. Up to the time that the limit is reached the distances a and b do not vary and the `pull of the lever I9 on the actuating rod 8| is an accurate measure of the applied force, being proportional to the pushing force of the jack-screw, irrespective of variations in torque that may be effected by friction. Upon attainment of the predetermined limit, say 1600 pounds on ,thejack-screw or 400 pounds on the actuating rodB I, the leverV I 9 starts to move clockwise about its pivotalconnection II8, pulling the actuating rod 8| -to .the right, thereby lifting plate 93 from its seat and compressing spring beyond the pre-loaded force. As soon as movement of the actuating rod 8| begins, the force pulling on the rod tends toincrease because the distance a increases thereby afecting the leverage. Almost immediately after the -actuating rod starts to move, however, it pulls the insulating block 89 to move the contact roller IUI out of engagement with the contact plate |92 arid-farther ionltoithe contact plate |93, thereby interrupting the circuit previously established through the contact plate |02. The motor will thereupon be immediately stopped.

IfV the switch arm |06 is moved to establish a circuit through contact plate |03, the rotation of the motor and jack-screw I4 is in the opposite direction... The pre-.loaded spring I'I will return' the contact rollerA IBI to its neutral position in which it bridges the gap between the contact plates |02 and |03 as the motor is rotated again in the opposite direction.

vIf the motor is rotated so that the jack-screw sleeve 2| is withdrawn, rotation will continue until lever arm I2 reaches its opposite limit position, represented diagrammatically by stop |2|. During such rotation the jack-screw I4 will move into the stationary sleeve 2|. A reactive force is transmitted through the shoulder 36, thrust bearing 32, housing 2l, gear case 3|, screws 3'4, bracket plate 33, bracket |2I, pivot pin II8, lever I9, pivot pin III and anchor bracket II. The cycle of operation is similar to the one described above in connection with the forward stroke. At the end of the return stroke and upon the force becoming excessive, the lever I9 is turned counterclockwise about pivot pin ||8 on bracket |2|.

The upper end of the lever I9 will act upon the actuating rod 8| to cause the plate 94 to further compress the pre-loaded spring II. In doing so the insulating block 89 will carry the contact roller out of contact with plate |03 thereby breaking the circuit which has been established through the contact plate |93 and stopping the motor. The roller IBI is returned to the gap when the motor is rotated in the opposite direction.

It is an essential feature of the invention that the lever I2 pivot about the pivot pin IIB when the housing 2l is moved by the jack-screw Ill. In other words, the upper end of the long arm of lever I9 will be moved to the right (Fig. 1) by the moving housing, and simultaneously will be rotated clockwise about the pivot pin IIB, which is xed to the housing by means of bracket |2|. By virtue of such movement the rod SI will be moved toward the right with respect to the housing, thereby actuating the switch member IUI to shut ol the motor. In case of movement of the housing to the left the upper end of the long arm of lever I9 will be moved to the left by the moving housing and simultaneously will be rotated about the pivot pin IIB, forcing rod 8| to the left with respect to the housing, thus actuating the switch -member IIBI to shut one the motor.

By arranging the spring to be actuated by a lever instead of simply placing it between the housing 21 and anchor bracket II, the spring need only sustain a fractional part of the thrust of the jack-screw, whereas, if the spring received the entire force, it would have to be made much larger.

It should now be apparent that there has been provided an actuator of the linear type which is simple in construction, and of compact design, which is not subject to maladjustment due to wear, and which directly measures the axial force within close limits and eliminates errors resulting from jack-screw friction.

While Various changes may be made in the detailed construction, it will be understood that such changes will be Within the spirit and scope of the present invention as dened by the appended claims.

What is claimed is:

l. A linear actuator comprising a frame, a housing movable in respect to the frame, a member adapted to be extended from and withdrawn into the housing, jack-screw means cooperating with said member to move the same, reversible electric motor means for operating the jackscrew, a bell crank anchored at one point to the Vframe and pivotally connected at an intermediate point to the housing, an enclosed pre-loaded spring device pivotally connected to the bell crank for maintaining the bell crank immovable with reference to the housing under normal load conditions of the actuator, means on the frame for limiting the extensible movement of the member and adapted; upon the continued application of motor power to the jack-screw after the limitation of such extensible movement to cause the jack-screw to move longitudinally away from the member thereby to pivot the bell crank on the frame anchor as a fulcrum and to vary the load of the pre-loaded spring, a limit switch mounted upon the motor and operated by movement of the spring and adapted to shut off the motor upon a` predetermined variance of the load of the spring. y

2. A linear actuator comprising a frame, a housing movable in respect to the frame, a member adapted to be extended from and withdrawn into the housing, jack-screw means cooperating with said member to move the same, a reversible electric motor means for operating the jackscrew thereby to cause the member to be extended from and withdrawn into the housing, a limit switch controlling the motor, a bell crank pivotally anchored to resisting means on the frame and pivotally connected at an intermediate point to the housing being thereby adapted to rotate about the anchor means and to pivot on the housing upon movement of the housing in respect to the frame, a pre-loaded spring device mounted upon the housing and cooperating ,at its extremities with the limit switch and the bell crank, said device being adapted to maintain the bell crank stationary with respect to the housing during the movement of extension and Withdrawal of the member from and into the housing, means on the frame to limit motion of the extensible member and adapted upon the continued operation of the jack-screw by the motor to cause the jackscrew to move longitudinally carrying the housing therewith thereby pivoting the bell crank on the housing, a rod extending through the spring and movable therewith which is connected at its ends with the limit switch and the bell crank, said rod being adapted upon movement of the bell crankto transmit a predetermined pressure from the bell crank to the limit switch to shut off the motor.

3. In an actuator, a frame, a housing movable with reference thereto, a jack-screw mounted in the housing, a sleeve threadably mounted on the jack-screw and movable axially with reference thereto, a reversible electric motor for effecting relative movement between the jackscrew and sleeve, a limit switch associated with the motor, a spring carried in the housing and preloaded to a predetermined compression, adjustable means on the housing to maintain such spring at predetermined compression, an actuating rod extending through the spring and adapted to operate the limit switch, plates loosely fitted on the rod and at opposite ends of the spring adjacent the spring compression maintaining means, means on the rod adapted upon longitudinal movement of the rod to transmit pressure through the plates to the spring, a lever pivotally mounted on resisting means on the frame and having connections at an intermediate point with the housing and at another point with the actuating rod, means upon the frame limiting the axial movement of the sleeve with reference to the jack-screw and adapted to cause the jack-screw upon its continued rotation in the sleeve after limitation of axial movement of the sleeve to move longitudinally away from such limiting means carrying the housing therewith and causing the lever to Ipivot and to exercise a predetermined pulling force upon the rod to actuate the limit switch thereby to shut off the motor.

4. An actuator according to claim 3 in which the sleeve is provided with a guide secured to the housing, said guide carrying wiper means for cleaning the sleeve lupon its movement in and out of the guide.

5. An actuator according to claim 3 in which the actuating rod is of adjustable length.

6. A linear actuator for use on aircraft comprising relatively movable members, means for connecting the members between a plurality of bodies for imparting relative movement therebetween including in the connection of at least one of the members, a movable third member, means including an enclosed pre-loaded compression spring for restraining the third member against movement during a predetermined range of movement between said relatively movable members but actionable to release the third member for movement when said movable members are actuated to exceed said predetermined range, power means operable for moving the members, and switch means mounted upon the power means and being responsive to the movement of said third member to discontinue operation of g said power means.

7. An actuator comprising a housing, a jackscrew mounted in said housing, a sleeve threadably mounted on the jack-screw and movable axially with reference thereto, a reversible electric motor adapted for eiecting relative movement between the jack-screw and sleeve, switch means mounted upon the motor, a helical spring carried in said housing, an actuating rod extending through and operatively associated with said spring and movably adapted to operate said switch means, said spring being compressively pre-loaded to restrain movement of said actuating rod until said jack-screw and said sleeve have attained relative movement of a predetermined amount, and a lever pivotally mounted on a xed support, exterior to said housing and being connected at one point to said housing, and at another point to said actuating rod, all being so arranged that upon attainment of said predetermined amount of relative movement between said jack-screw and said sleeve, an exterior restraining force will become operative so that further action between said jack-screw and said sleeve will cause said spring to be further compressed,

resulting in the movement of said actuating rod to operate said switch means to discontinue operation of said motor.

8. A linear actuator comprising a frame, a housing movable in respect to the frame, a member adapted to be extended from and withdrawn into the housing, jack-screw means cooperating with said member to move the same, a reversible electric motor means for operating the j ack-screw thereby to cause the member to be extended from and withdrawn into the housing, a limit switch controlling the motor, a lever pivotally anchored to resisting means on the frame and pivotally connected to the housing being thereby adapted to rotate about the anchor means and to pivot on the housing upon movement of the housing in respect to the frame, a pre-loaded spring device mounted upon the housing and cooperating with the limit switch and the lever, said spring device being adapted to maintain the lever stationary with respect to the housing during the movement of extension and withdrawal of the member from and into the housing, means on the frame to limit motion of the extensible member and adapted upon the continued operation of the jack-screw by the motor to cause the jack-screw to move longitudinally carrying the housing therewith thereby pivoting the lever upon the housing, a rod extending through the spring and movable therewith being connected with the limit switch and the lever, said rod being adapted upon movement of the lever to transmit a predetermined movement from the lever to the limit switch to shut off the motor.

9. A linear actuator according to claim 7 wherein said switch means comprises a contact roller, a grounded contact spring, and a pair of contact plates mounted yupon a fixed insulation plate, said roller being maintained in a movable insulation member affixed to the actuating rod, and so arranged that in neutral position the roller bridges the contact plates and simultaneously engages the grounded contact spring, and further arranged that in either of two limiting positions of said movable insulation the roller breaks contact with one of the contact plates.

LESTER A. AMTSBERG. JOHN E. OLSON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,945,496 Sloan et al. Jan. 30, 1934 2,175,047 Acker Oct. 3, 1939 2,259,437 Dean Oct. 21, 1941 2,317,490 Simpson Apr. 27, 1943 

